1. Technical Field of the Invention
The present invention relates to a cage-type induction motor that is appropriate for and used for high rotational speeds.
2. Prior Art
When a rotatable disk made of copper is placed between the poles of a permanent magnet shaped like a cradle and the magnet is rotated in one direction, the disk revolves in the direction of movement of the magnet. This phenomenon is called Arago""s rotating disk and provides the operating principle of an induction motor. As the magnet moves, a current is induced in the disk (Fleming""s right-hand rule), and a rotational force is produced by the current and the magnetic flux of the magnet (Fleming""s left-hand rule), and the disk rotates in the same direction as the movement of the magnet.
In a polyphase induction motor, the movement of the permanent magnet in Arago""s rotating disk is replaced by a rotating magnetic field produced by the polyphase alternating current, and the motor is composed of a stator that produces the rotating magnetic field and a rotor that rotates. Unlike Arago""s rotating disk, the direction of magnetic flux in a motor is arranged so that the direction of the magnetic flux is perpendicular to the rotor surface, and the direction of the currents induced in the rotor is parallel to the shaft. Therefore, the stator and the rotor are coaxial cylinders.
The stator generates a rotating magnetic field using AC electric power from a polyphase power supply, induces a current in the secondary winding of the rotor by induction through the gap, a rotational force is produced by the current and the magnetic flux of the rotating magnetic field (Fleming""s left-hand rule), and the rotor rotates in the same direction as that of the rotating magnetic field.
The stator is normally composed of an iron core and a stator winding, both housed in the stator frame. The iron core of the stator is made of thin steel sheets laminated in the axial direction to reduce iron losses. The stator winding is placed in slots in the iron core, connected to a polyphase power supply and produces a rotating magnetic field.
The rotor is normally composed of a laminated iron core (rotor core) and a rotor winding. The rotor winding is installed in the slots of the iron core. The rotors are classified as either cage type or winding type. In a cage-type rotor, a copper bar is installed, in each slot of the rotor, and both ends of the copper bars are connected together by end rings.
When a cage-type induction motor rotates at a high speed (for instance, 100,000 rpm or more) and, for example, directly drives and rotates a turbo compressor, to enhance the reliability of the equipment and make it compact and reduce the power consumption various conditions are necessary including (1) a rigid structure capable of withstanding a high peripheral speed, (2) high efficiency and (3) high power factor.
In the conventional cage-type induction motor shown in FIG. 1A, laminated steel sheets are normally used in the rotor core 1, the conductors 2 are connected into a cage, the teeth 4a of the stator 3 are made partly open, and the air gap is small (0.5xcx9c1% of the diameter of the rotor).
However, this structure of a cage-type induction motor known in the prior art has problems as it severely limits the peripheral speed and causes local stress concentrations. More specifically, since the stresses are concentrated in the center part of the rotor-core 1, composed of laminated steel sheets, due to the centrifugal forces caused by the high rotational speed, the peripheral speed must be limited to, for example, about 200xcx9c230 m/s to prevent the rotor core from being fractured. Therefore, this type of induction motor is not suitable for higher rotational speeds. In addition, because part of the conductor 2 protrudes through the rotor surface, there is the problem of stress concentrations in the thin-wall portions of the cage of the rotor core.
To solve this problem, the patents xe2x80x9cCage-type rotor of high-speed induction motorxe2x80x9d (unexamined Japanese patent publication No. 253511, 1994), xe2x80x9cSolid rotor for cage-type induction motor and its manufacturing methodxe2x80x9d (unexamined Japanese patent publication No. 127022, 1998), etc. proposed a rotor with a structure such as that shown in FIG. 1B, the laminated steel sheets are replaced by a solid rotor (integrated) to increase the strength, and the cage bars are embedded for protection.
However, the aforementioned solid, embedded cage rotor suffers from the problems that (1) the electrical conductivity at the rotor surface is high, and eddy currents are produced on the rotor surface, (2) the eddy currents on the surface do not contribute to rotating torque, but reduce the efficiency due to eddy current losses, (2) the effects of the eddy currents are concentrated at the end portions of the stator teeth, where the magnetic flux is not distributed uniformly, and so on.
Also the patent, xe2x80x9cAsynchronous electric machine and rotor and stator for use in association therewithxe2x80x9d (U.S. Pat. No. 5,473,211) proposes the structure shown in FIG. 1C wherein the conductors are disposed continuously on the surface of the rotor, and the air gap is large. More explicitly, this patented invention provides an integrated structure of the rotor by coating the entire surface of the rotor with a high electrical conductivity material so that rotational speeds as high as a maximum of 1 million rpm are possible and by making the gap xcex4 between the rotor and the stator greater than the conventional value (0.5xcx9c1% of the diameter of a rotor), the harmonic components in the distribution of magnetic flux are reduced by the large gap, resulting in a reduction of the eddy current losses.
However, the above-mentioned continuously coated surface rotor structure cannot avoid problems such as (1) because the gap 6, xcex4, is large and the surface of the rotor is covered with a coating of high electrical conductivity material, the distance from the inner surface of the stator to the magnetic material of the rotor core (thickness of the surface coating+air gap) is increased, causing an increase in the inactive magnetic flux, so that the power factor is reduced, and (2) since the electrical conductivity on the surface of the conductor is uniform, eddy currents are produced in the surface coating.
The present invention is aimed at solving these problems. That is, the objective of the present invention is to provide a cage-type induction motor for high rotational speeds with the advantages that (1) high rotational speeds are enabled by integrating the structure of the rotor and eliminating local stress concentrations, (2) the efficiency is increased by reducing the formation of eddy currents on the rotor surface, and (3) the power factor can be improved by shortening the distance between the inner surface of the stator and the magnetic material of the rotor core, thereby reducing the inactive magnetic flux in the gap.
According to the present invention, in an induction motor using a stator that produces a rotating magnetic field and a rotor that rotates, the aforementioned rotor (10) is composed of a rotor core (12) and rotor conductors (14), the rotor core is made of a weakly magnetic material with a high permeability and a relatively low electrical conductivity, the rotor conductors are composed of a highly conducting substance with a low permeability and a relatively high electrical conductivity, the aforementioned rotor core (12) and rotor conductors (14) are integrated together into a smooth uniform cylindrical surface, the above-mentioned stator (20) is formed from a plurality of stator sheets (22) laminated in the axial direction and a stator winding (24), the shape of each stator sheet is such that there is a circular, closed inner ring portion (22a) and an outer ring portion (22b), there are slots (23) penetrating the sheets, and the stator winding is contained in the aforementioned slots.
In the aforementioned configuration according to the present invention, the rotor core (12) and the rotor conductors (14) are integrated into one body and the outer surface is a smooth cylindrical surface, therefore, there are no stress concentrations and the centrifugal stresses can be transmitted smoothly to the rotor core which has a high tensile strength, and the rotor can withstand a high peripheral speed.
The inner ring portion (22a) of the axially laminated stator sheets (22) and the outer ring portion (22b) individually form two smooth surfaces without gaps, and the longitudinal slots (23) in which the fixed winding is installed are between the two surfaces, therefore the slots do not open onto the surfaces, and the density of the magnetic flux passing from the inner ring portion (22a) into the circumferential gap is evenly distributed, so there is a reduction in the eddy currents generated in the rotor surface.
According to a preferred embodiment of the present invention, a plurality of teeth portions (22c) extend radially between the aforementioned slots (23) and connect the above-mentioned inner ring portion (22a) and outer ring portion (22b), and the stator winding is installed in the slots only at the teeth portions, and either the inner ring portion (22a) or the outer ring portion (22b) is joined to the other portion after the winding has been installed.
According to this configuration, the stator winding is installed in the slots only at the teeth portions while either the inner ring portion (22a) or the outer ring portion (22b) is removed, and the outer ring portion (22b) or the inner ring portion (22a) is placed in position after completing the installation of the winding, thus a stator without open slots can easily be manufactured.
Other objectives and advantages of the present invention are revealed in the following paragraphs referring to the attached drawings.